FIG. 18 is a block diagram showing a control apparatus for an electric motor according to related art to the present invention. The control apparatus for the electric motor 10 shown in FIG. 18 has a resolver 101, a current sensor 103, a band pass filter (BPF) 105, a three-phase-dq conversion section 107, a current command calculating section 109, a d-axis current control section 111, a q-axis current control section 113, an rθ conversion section 115, an inverter (INV) 117, an angular velocity calculating section 119, a direct voltage command generating section 121, a DC-DC converter 123, an output voltage detecting section 125, and an inverter control method determining section 127. In the electric motor 10 shown in FIG. 18, electric power is supplied from the condenser 15 through the control apparatus. Here, the electric motor 10 is, for example, a three-phase brushless DC electric motor provided with a rotator having a permanent magnet and with a stator for generating a rotating magnetic field by the applied three-phase voltages and thereby revolving the rotator.
The resolver 101 detects a mechanical angle of the rotator of the electric motor 10, and outputs an electrical angle θm corresponding to the detected mechanical angle. The electrical angle θm outputted from the resolver 101 is transmitted to the three-phase-dq conversion section 107 and the angular velocity calculating section 119. The current sensor 103 detects each phase current of the three-phase currents outputted from the inverter 117 and supplied to the stator of the electric motor 10.
The BPF 105 removes unnecessary components in the current detection signals indicating the three-phase alternating currents Iu, Iv, and Iw detected by the current sensor 103. The three-phase-dq conversion section 107 performs three-phase-dq conversion based on the current detection signals in which unnecessary components have been removed by the BPF 105 and the electrical angle θm of the rotator detected by the resolver 101, so as to calculate a detected value Id_s of the d-axis current and a detected value Iq_s of the q-axis current.
Based on a torque command value T inputted from the outside, the current command calculating section 109 determines a command value Id* for a current (a “d-axis current”, hereinafter) to be supplied to the stator (a “d-axis stator”, hereinafter) on the d-axis side and a command value Iq* for a current (a “q-axis current”, hereinafter) to be supplied to the stator (a “q-axis stator”, hereinafter) on the q-axis side. The command value Id* for the d-axis current is inputted to the d-axis current control section 111. Further, the command value Iq* for the q-axis current is inputted to the q-axis current control section 113. Here, the d-axis is the field axis, and the q-axis is the torque axis.
The d-axis current control section 111 determines the command value Vd** for a terminal-to-terminal voltage (a “d-axis voltage”, hereinafter) for the d-axis stator such that the deviation ΔId between the command value Id* and the detected value Id_s for the d-axis current should decrease. The q-axis current control section 113 determines the command value Vq** for a terminal-to-terminal voltage (a “q-axis voltage”, hereinafter) for the q-axis stator such that the deviation ΔIq between the command value Iq* and the detected value Iq_s for the q-axis current should decrease. The command value Vd** for d-axis voltage and the command value Vq** for the q-axis voltage are inputted to the rθ conversion section 115 and the inverter control method determining section 127.
The rθ conversion section 115 converts the command value Vd** for the d-axis voltage and the command value Vq** for the q-axis voltage into components of a voltage level V1 and an angle θ.
Based on the components of the voltage level V1 and the angle θ inputted from the rθ conversion section 115, the inverter 117 converts the direct voltage from the condenser 15 through the DC-DC converter 123, into alternating voltages of three phases (U, V, an W). Here, the inverter 117 is a rectangular wave inverter, and performs PWM (Pulse Width Modulation) control or alternatively one pulse (1 PLS) control depending on a switching flag inputted from the inverter control method determining section 127. Here, in the PWM control, a higher switching frequency permits the control of the output voltage of the inverter 117 with higher precision. On the other hand, in the 1 PLS control, the switching frequency is low and hence a switching loss is small.
The angular velocity calculating section 119 performs time differentiation of the electrical angle θm outputted from the resolver 101, so as to calculate the angular velocity ω of the rotator of the electric motor 10. The angular velocity ω calculated by the angular velocity calculating section 119 is inputted to the direct voltage command generating section 121.
The direct voltage command generating section 121 refers to a table describing a correspondence between the angular velocity ω and the output voltage command Vcu*, and thereby generates an output voltage command Vcu* for instructing the DC-DC converter 123 to output a fixed direct voltage corresponding to the angular velocity ω inputted from the angular velocity calculating section 119. The output voltage command Vcu* is inputted to the DC-DC converter 123. The DC-DC converter 123 raises or lowers the output direct voltage of the condenser 15 in the intact direct-current form. The output voltage detecting section 125 detects the output voltage Vdc of the DC-DC converter 123.
Based on the output voltage Vdc of the DC-DC converter 123 as well as the command value Vd** for the d-axis voltage outputted from the d-axis current control section 111 and the command value Vq** for the q-axis voltage outputted from the q-axis current control section 113, the inverter control method determining section 127 determines a switching flag to be inputted to the inverter 117.
FIG. 19 is a block diagram showing an internal configuration of the inverter control method determining section 127 and a relation with components relevant to this. As shown in FIG. 19, the inverter control method determining section 127 has a maximum voltage circle calculating section 201, an output voltage circle calculating section 203, and a switching flag output section 205. The maximum voltage circle calculating section 201 derives a value Vp_target (Vdc/√6) obtained by dividing by √6 the output voltage Vdc of the DC-DC converter 123. This value Vp_target is the maximum of the phase voltage allowed to be applied onto the electric motor 10, that is, a phase voltage value applied onto the electric motor 10 in a state that the duty ratio in the inverter 117 is 100%.
The output voltage circle calculating section 203 derives the calculation result of √(Vd**2+Vq**2) as a resultant vector voltage Vp. The switching flag output section 205 outputs a switching flag corresponding to the difference ΔVp (=Vp_target−Vp) between the value Vp_target derived by the maximum voltage circle calculating section 201 and the resultant vector voltage Vp derived by the output voltage circle calculating section 203. When the difference ΔVp is greater than 0 (ΔVp>0), the switching flag output section 205 outputs a switching flag specifying PWM control, and when the difference ΔVp is smaller than or equal to 0 (ΔVp≦0), outputs a flag specifying 1 PLS control.